Various vehicle tire air pressure detection systems are proposed. In JP P2000-255229A, for instance, a transmitter and an air pressure sensor are attached to a vehicle wheel and operated with electric power supplied from a vehicle chassis side in a contactless manner, that is, by electromagnetically coupling coils of the vehicle chassis side and the vehicle tire side. In this system, if the coils are electromagnetically operating to supply the electric power, it is likely that an air pressure detection data transmitted as an electromagnetic wave from the transmitter is interfered by the electromagnetic operation of the coils.